A conventional brushless direct current motor as used on a textile machine includes a three-phase wire wound stator, a permanent magnet armature, and a fixed encoder assembly coupled to the magnetic signal of the armature. The encoder assembly provides logic level signals to indicate motor armature position.
The brushless direct current motor has a power supply in the form of a power transistor switching circuit which is designed to provide current flow through the three stator windings in sequence. The direction of current flow is controlled by a signal from the encoder assembly.
The current flow through the stator windings causes an incremental rotation of the armature and the encoder, deriving a magnetic signal from the armature, which in turn causes the power circuit to switch the current flow at appropriate positions of the armature to continue the rotation of the armature. The continuous switching of current flow through the stator windings, commonly referred to as commutation, supports the rotation of the motor armature. This is similar to a conventional direct current motor with fixed carbon brushes and a rotating commutator in the form of copper segments attached to lead ends of the armature windings.
The brushless motor power supply conventionally includes six power transistors. At any one instance during armature rotation, only two of the six transistors are conducting. The total current flow to the motor is supported by all six transistors, and each of the transistors which are connected in series need only be selected to support one-third of the total motor current.
Brushless direct current motors are particularly useful in a textile machine, such as a take-up device as described in Benjamin, United States application Ser. No. 609,113 filed May 11, 1984; and in textile feeders because of their small size and the ability to operate such motors in hazardous environments. However, in a take-up device as described in the Benjamin application, it is necessary in operation of the device to lock the spindle driven by the motor to doff and don yarn packages. In textile feeders oftentimes a yarn strand becomes entangled which can accidentally lock-up the motor. In such machine applications of the motor which cause the motor armature to be clamped in a stall condition, the armature current flow will be concentrated through only the two transistors which are selected by the encoder for that particular armature position. The conventional control circuit will cause full current to flow through these two transistors. This will represent a demand upon these two transistors which will be three times normal requirement.
It will be readily apparent that the selection of transistors capable of receiving three times normal current requirement represents a great addition to the power supply course. It is, therefore, the object of this invention to eliminate the requirement for such high capacity transistors by implementing an alternative stategy in the control circuit design.
It is to be understood that the condition in which the motor is clamped or stalled is an undesirable or unusual machine condition. Detection of the stall condition can appropriately be utilized for purposely faulting or shutting down the operation of the power supply.